Q.42 Given below are two statements : One is labelled as Assertion (A) and the other is labelled as
Reason (R).
Assertion (A) :Constitutive hetecochromatin is the region of chromosome, that is always
heterochromatic and pesmanently inactive with regard to transcription.
Reasons (R) :Facultative heterochromatin refers to chromatin that can occasionally
interconvert between heterochromatin and euchromatin.
In the light of the above statements, choose the most appropriate answer from the options given
below :
(1)Both (A) and (R) are correct and (R) is the correct explanation of (A)
(2)Both (A) and (R) are correct but (R) is NOT the correct explanation of (A)
(3)(A) is correct but (R) is not correct
(4)(A) is not correct but (R) is correct
Constitutive and Facultative Heterochromatin: Assertion-Reason Analysis for Competitive Exams
Constitutive heterochromatin remains permanently condensed and transcriptionally inactive, while facultative heterochromatin can switch states. Both statements are correct, but unrelated, making option (2) the right choice.
Question Breakdown
The assertion correctly defines constitutive heterochromatin as always heterochromatic and permanently inactive for transcription, typically containing repetitive DNA like centromeres and telomeres.
The reason accurately describes facultative heterochromatin as capable of interconverting between heterochromatin (inactive) and euchromatin (active) states, such as in X-chromosome inactivation.
These are two distinct types of heterochromatin, so (R) does not explain (A).
Option Analysis
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Option (1): Wrong—Both true, but (R) does not explain why constitutive heterochromatin is always inactive; it describes a different chromatin type.
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Option (2): Correct—Both statements are true independently; constitutive is fixed, facultative is dynamic.
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Option (3): Wrong—(R) is true, as facultative heterochromatin reversibly changes states under developmental cues.
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Option (4): Wrong—(A) is true, matching standard definitions of constitutive heterochromatin’s permanent inactivity.
Key Differences
| Feature | Constitutive Heterochromatin | Facultative Heterochromatin |
|---|---|---|
| State Permanence | Always condensed, inactive | Can interconvert to euchromatin |
| DNA Content | Repetitive sequences | Unique genes (e.g., Barr body) |
| Example | Centromeres, telomeres | Inactive X chromosome |
| Transcription | Permanently off | Occasionally active |
